What does ?: do in Kotlin? (Elvis Operator)

Question

I can’t understand what is doing ?:, for example, in this case

val list = mutableList ?: mutableListOf() 

and why it can be changed to this

val list = if (mutableList != null) mutableList else mutableListOf()

Answer 1

TL DR: if the resulting object reference [first operand] is not equal null, it is returned. Otherwise, the value of the second operand (which may be null) is returned

---

The Elvis operator is part of many programming languages, for example, Kotlin, as well as Groovy or C #. I find the definition of Wikipedia fairly accurate:

?: , , true, . ? : ? :, ( ): - , .

Kotlin:

. , , . null, . ( null).

:

x ?: y // yields 'x' if 'x' is not null, 'y' otherwise.

Answer 2

, : ?: :

first operand ?: second operand

:

first operand , . , second operand. , , , .

---

( ):

fun retrieveString(): String {    //Notice that this type isn't nullable
    var nullableVariable: String? = getPotentialNull() //This variable may be null

    return nullableVariable ?: "Secondary Not-Null String"
}

, getPotentialNull , retrieveString; , "Secondary Not-Null String".

, , .

Kotlin second operand, , throw Exception

return nullVariable ?: throw IllegalResponseException("My inner function returned null! Oh no!")

---

Elvis Operator .

: , . , . Android Kotlin. 2017. Packt Publishing

Answer 3

Elvis operator, ... , . null, , . .

a ?: b if (a != null) a else b.

:

val x: String? = "foo"
val y: String = x ?: "bar"      // "foo", because x was non-null    

val a: String? = null
val b: String = a ?: "bar"      // "bar", because a was null

Answer 4

:

r, " r , , x":

?: (Elvis) .

, null .

listOf(1, 2, 3).firstOrNull { it == 4 } ?: throw IllegalStateException("Ups")

?: , . , , :

val l = listOf(1, 2, 3)

val x = l.firstOrNull { it == 4 } ?: l.firstOrNull { it == 5 } ?: throw IllegalStateException("Ups")

if else, , .

Answer 5

, . , ? , return empty busy

Java:

private int a;
if(a != null){
    println("a is not null, Value is: "+a)
}
else{
    println("a is null")
}

:

val a : Int = 5
val l : Int = if (a != null) a.length else "a is null"

Answer 6

, - , .

..

val number: Int? = null
println(number ?: "Number is null")

Thus, if the number is NOT equal to zero , a number will be printed, otherwise, "Number is zero" will be printed.